The circulatory network in mammals is composed of a cardiovascular and lymphatic system responsible for delivering oxygen, nutrition, immune cells, and hormones to tissue and collecting waste materials from cells within living organs. These compounds are exchanged with cells via capillary beds where, due to the high blood pressure at arterioles, some part of plasma leaks into the interstitial space. After the exchange, the protein-rich interstitial fluid (lymph) is collected and returned back to the circulatory system by the lymphatic system. The lymphatic system consists of unidirectional, thin-walled capillaries, and a larger vessel network that drains lymph fluid from extracellular spaces within organs into larger collecting ducts. Finally, lymph is returned to venous circulation through the thoracic duct. The lymphatic system also includes lymphoid organs such as lymph nodes, tonsils, Peyer's patches, spleen, and thymus which play a crucial part in immune response. The lymphatic system usually develops in parallel to the blood vessels in the skin and in most internal organs and is not present in the central nervous system, bone marrow, and avascular structures such as cartilage, epidermis, and cornea. Besides draining lymph fluid from extracellular spaces, other roles of the lymphatic system include absorbing lipids from the intestinal tract, maintaining the fluid hemostasis and transporting antigen-presenting cells and leukocytes to lymphoid organs. Also, the lymphatic system plays an important role in the development of several diseases such as cancer, lymphedema, and some inflammatory conditions.